Bushings are devices that can carry current at high potential through a grounded barrier, e.g., a transformer tank. In order to decrease and control the electric field near the bushing, condenser bushings have been developed, also known as (fine-) graded bushings. Condenser bushings can facilitate electrical stress control through insertion of floating equalizer (electrode) plates, which are incorporated in the core of the bushing. The condenser core can decrease the field gradient and distribute the field along the length of the insulator, which can provide for low partial discharge readings well above nominal voltage readings.
The condenser core of a bushing can be wound from kraft paper or creped kraft paper as a spacer. The equalization plates can be constructed of either metallic (typically aluminium) inserts or nonmetallic (ink, graphite paste) patches. These plates can be located coaxially so as to achieve an optimal balance between external flashover and internal puncture strength. The paper spacer can ensure a defined position of the electrodes plates and provide for mechanical stability.
The condenser cores of today's bushings are impregnated either with oil (OIP, oil impregnated paper) or with resin (RIP, resin impregnated paper). RIP bushings have the advantage that they are dry (oil free) bushings. The core of an RIP bushing is wound from paper, with the electrode plates being inserted in appropriate places between neighboring paper windings. The resin is then introduced during a heating and vacuum process of the core.
The process of impregnating the paper with oil or with a resin can be slow process.